Process for the recovery of oxygen from gaseous mixtures



Oct. 15, 1940. K. c. WARNE ET Al. 2,217,850

PROCESS FOR THE RECOVERY 0F OXYGEN FROM GASEOUS MIXTURES Filed May 18, 1939 Kawai/2 C Wr/2@ c/azzzef W M/OOZCCZ( INVENToRs ATTORNEY Patented Oct. 15, 1940 PATE.

NT fol-"Fics PROCESS FOR THE RECOVERY-OF OXYGENl FROM GAsEoUs MIXTURES Kenneth Cecil Warne and James William Woolcock, Norton-on-Tees, England, assignors to Imperial Chemical Industries Limited, a corporation of Great'ritain Application May 18, 1939, -Serial No. 274,336

In Great 18 Claims.

This invention relates to the recovery of oxygen from gaseous mixtures containing it.

It has long been known that solutions of cobaltous hexammine salts take up oxygen on exposure to air, with the formation of compounds known as ozo or peroxo-compounds, and that a portionof the oxygen taken up is evolved when the solution is heated. The remainder of the oxygen is used up in oxidizing some of the cobaltous salt to the cobaltic state, in which state the salt is no longer capable of taking up oxygen.

Attempts have already been made to develop a commercial process for the recovery of oxygen from gaseous mixtures by repeated use and regeneration of a solution of cobaltous hexammine salt, and various soluble salts have been tried with a view to finding the mostsuitable one for alternately taking up and giving olf oxygen. In all cases, however, it was found that a steady fall of the absorptive power of the solution occurred on repeated use, due mainly to loss of cobaltous salt by oxidation to the cobaltic state in each successive cycle. To overcome this difficulty it was suggested that the cobaltic salt should be reduced electrolytically, but it was found that owing to ther low concentration of cobaltic ions in the solution the current eflicency was far too low for economic working, While if the cobaltic ion concentration were allowed to rise with a view to improving the current efficiency undesirable precipitation took place.

No simple chemical method of recovering the cobaltous salt from the cobaltic salt has been found.

This invention has-as an'ob-J'ect to devise a new process for the absorption and recovery of oxygen from oxygen-containing gases. A further object is to devise such a process wherein complex cobaltous salts are used. A still further objectis to devise such a process whereby the tendency of the complex salts is substantially eliminated. Further objects will appear hereinafter. 'Ihese objects are accomplished bythe following invention.

We have found that if We use, as anvabsorbent for the oxygen, a complex cobalt compound which is capable of exchanging part of its complex forming radical for oxygen and vice versa, and bring about the absorption and regeneration by changing the concentration of the complex forming radical in the aqueous medium and/or by changing the partial pressure of oxygen above 'the aqueous medium then the oxidation of the cobaltous compounds to cobaltic can be substantially eliminated. I Y

cobaltous salts to oxidize to cobaltic Britain August 23, 1937 While it is animportant feature of the invention that regeneration of the originalcobaltous saltcan be effected without raising the temperature, so that undesirable oxidation to the. cobaltic state is minimized or eliminated, temperature variations may be used in the process of the present invention to assist in the reversal of the absorption and regeneration steps, but the temperature must always be maintained below that at which the appreciable oxidation to the cobaltic state takes place, a condition which was not possible in the known processes, inwhich temperature variations were relied upon lfor reversal of the absorption-and regeneration steps, The highest permissible working` temperaturewill, ofcourse, depend upon the nature of the cobaltous salt and of the aqueous medium employed, e. g. with ammonia as the complex forming radical the vtemperature-should not be -.raised above 35 C. In general, operation at room temperature gives good results.'y .y

Since the essence of this invention is the utilization of the equilibrium between the complex co- -baltous salt and oxygen on .the rone side, and peroxo compound and freecomplex-forming radical onlthe other,.the equilibrium should 'be such that it can be reached easily under operating conditions of temperature and pressure from both sides.

. It will be obvious that it is desirable that .the equilibriumshould be capableof being reached from both sides Without excessively high pressures or inconveniently high concentrations of complexforming radical.

In order that the reaction should goto completion lor substantial completion in both directions with a small change in oxygen'partial pressure and/orconcentratin of complex-forming radical, the complex cobaltous salt or'the peroxo compound, and preferably both should be largely present in the Vsolid phase and preferably should have a sparing solubility in the aqueous medium or media, or should be capable ofI being made sparingly :soluble by the addition of alsalt having the'same acid radical as the complex cobaltous salt `and the peroxo compound.

, Themethod of the present inventionflargely overcomes the difficulty of irreversible oxidation tothe cobaltic state since it does not relyprimarily upon heating to eilectf regeneration. 50 Nevertheless since complex cobaltous salts yary' enormously in their tendency to u'ndergo some irreversible oxidation in the presenceof aqueous media even at ordinaryjtemperatureait is clearly desirable to select a salt Which undergoes only an liti` 2. extremely slight oxidation at ordinary temperatures. Complex cobaltous salts of strong acids appear to be preferable.

When using a sparingly soluble complex coregeneration stage medium by any suitable method, and fresh complex-forming radical may be added to make up for any losses in the process. Any complex-forming radical contained in the baltous compound and/or sparingly soluble peroxo gases leaving the process may be recovered and 5 compound, it is preferable to have dissolved in eturned to the absorption or regeneration stage the aqueous medium a salt of the acid from which medium. f the complex cobaltous salt is derived, as the pres- It will be understood that in the absorption ence of such a salt appears to assist in preventing stage the invention is not coniined to the use of undesirable oxidation to the cobaltic state. The one aqueous medium, and that a series of aqueous 10 salt may conveniently be a salt of the complexmedia of different complex-forming radical conforming radical. l centrations may be used. Similarly, in the re- The most suitable salt known to us at present generation stage instead of one aqueous medium, is cobaltous hexammine perchlorate, with aquea series of aqueous media may be employed.

l5 ous ammonia, preferably containing some dis,- i The absorption and regeneration stage media 15 solved ammonium perchlorate, as aqueous memay With advantage contain a compound which dium. As far as our investigations .have gone. it buffers the concentration of the complex-forming appears that perchlorate compounds give the best radical in these media. It is thereby rendered results, and the use of these compounds in proc- Possible t0 use a lOWer ratio 0f medium tosusesses of the kind described is believed to be broadly Dended CObaltOuS Complexl ComPOund Or PeroXo 20 novel. Under the .conditions of loperation co- Compound sine@ the presence of the buffer in the baltous hexammine perchlorate and its `peroxo absorption Stage medium'preVentS an undesirable COmpOund are only sparingly soluble in the aquerise in concentration of the free complex-forming ous medium so that the amount of aqueous meradical, which would sloW down substantially the 2 5 dium employed relative to the amount of Soiid rate of absorption, While in the regeneration stage 25 material may vary Within wide limits.- In genmedium, the 'Presence Of-the buffer Would loreVent eral, neither the amount nor the nature of the undesirable decrease in the concentration of the aqueous medium` need be the same in the two free complex-forming radical, which wouldv slow stages. down substantially the'rate 'of' regeneration.

In the presence of the aqueous mediumthe In addition, the presence of a buffer for the 30 complex cobaltous salt and its peroxo Compound coinplex-iorming radical in the absorption stage will usually tend to decompose with liberation medium renders it possible to Ohtain more easily of one or more molecules of the complex-forming a transfer of the freev COInDleX-'Ormrlg radical radical, e. g. ammonia. Such decomposition may Whioh aooumulates in the absorption Stagexmedibe prevented, howevery by having ammonia, or um back to the regeneration stage medium. This 35 other complex-forming radical dissolved in the is paftioulally advantageous When distillation is aqueous medium, v employed to effect the transfer, since the loose It is also an advantage to employ vigorous stir- Compounds of the buffer and eompleX-olrning ring in the absorption Stage ospeeiaiiy when the radical dissociate and increasethe concentration 4o complex cobaltous compound is sparingly soluble, o ieev Complex-forming radieal in theaqueous 4o and in the regeneration stage especially When the medium, thereby giving a higher ratio 0f Comperoxo oompoundis sparingly so1ubioplex-forming radical to water in the distillate.

We have also found that the presence of one The Variation in Concentration of COInPleX- 0r more pOlyhydriC alcohols, e. g.- ethylene giyooi forming radical may also be obtained otherwise in the aqueous medium or media used in regenthan by the physical transference of quantities 45 eration increasesthe'rate offregeneration. o media- One aqueous medium may' be used Increase of tneeonoentrotion of complexfor both absorption and regeneration stages. In forming radical. in the aqueous medium for the this Case, ammonia 0r Other gaseous or vaporised purpose of regeneration may be effected in any complex-,forming radical may be introduced intO 504 convenient manner. i For example, When both the o TemoVed from the aqueous mediumfas a gas, complex cobaltous salt .andits peroxo compound oi the aqueous medium may Contain a SubStanCe are sparingly soluble in the aqueous media em- 01 substanties Capable of giVing rise tohange ployed, the absorption Stage may be earned out of concentration of complex-forming radioalunwith the solid complex cobaltous salt in contact der the Conditions of Wolking- Thus a hydrolysawith a quantity of medium .relatively poor in the ble ammonium salt may loe..pl"esent Whiohon 55 complex-forming radical, and regeneration then hydrolysis (faVoured hy raising 'the temperature obtained by replacing the saidmedium by a quan- 0r' otherwise) gives rise to an insoluble 0r Subtity of 'medium relatively rich in thek comp1exstantially un-ionised acid or a volatileacidwhich forming radical. During the absorption stage the Can he remoVed fr om the 4aqueous medium, With 60. complex cobaltous salt rejects some complexthe result that the concentration of free ammonia forming raoicai'to tnemedium in Contact with in the aqueous medium may be increased or deit, While during the regeneration stage lthe per- Creased at Willoxo-compound takes up some complex-forming Ammonium loorate is an example of a soluble radical from the medium ziu- Contact, VWith je salt which has a high coeiiicientoiv hydrolysis in Thus, in effect, complex-forming radical is transthe permissible Working range 0f temperature. It 65 ferred from the regeneration stage medium to is Possible to get an @Ven greater Change in free the absorption stage. mediumby the complex coammonia Concentration over a' Small range of baltous salt andV its peroxo compound,v and to temperature by taking advantage of the high enable the medium to. be repeatediy used some temperature coefcient of solubilityof lithium A complex-forming radical has to be removed from metahoTa-te ootahydrate used in Conjuneton With 70 'the absorption lstage medium and fresh complexan ammonium salt of' a strong aeid, for eXample, forming radical introduced into theregeneration ammonium sulphate At loWeI` temperatures the stage medium, after each cycle or at convenient tendency is for lithium rnetaborate octahydrate intervals. Complexfformingradical may be reto be present as a solid, and the free ammonia x f5- turned from theY absorption stage medium to the concentration in the aqueous mediumto be low', 75

temperature.

`the lithium metaborate octahydrate which there- -by crystallisesout cannot occlude any particles of the cobaltous hexammine compound for the peroxo compound. f y

The process maybe Vfurther modiedrbychoos'- ing an acid radicalsuch that theisolubility of its lithium compound' does not :increase rapidlyW-ith temperature. It is then possible by suitable. choice of aqueous r'nediumand concentrations to crystallise out the lithiumsalt 'of the strong .acid at; the higher temperature with a viewto obtaining.. an even greater change of freeI ammonia concentration over a limited. temperaturerange..

Salts which give'rise to'. ammoniaorothercomplex-formingradicalV on heating .may also' .be

employed to produce variations of concentration of complex-forming radical'in the aqueous me,-`

dium. Another possibilityis tohave presenta basic ammonium saltor compound the concentrationof which in solution changes rapidly with The extent ofthe variation in ooncentrationof complex-forming radical in theaqueous medium required in the process depends upon severalfactors, including temperature, oxygen ,partialV pressure, and the nature of the complexA` cobaltous salt, its peroxo compound and-theaqueous medium or media, andthe rate of reaction desired, but the variation forany given set of operating conditions can easily be determinedby Ordinary The invention is illustrated in the folivvingx; ample which refers to the accompanying drawing.

- Example v The aqueous suspensiony vemployed wasvmade by mixing 61 parts by weight of cobaltous'perchlorate hexahydrate (Co (ClO4) 2.6H2O) .with an aqueous solution-containing@ parts by Weight of ammonium perchlorateiNI-IiClOr), 17.141 parts by weight of ammonia and 400 parts by weight of water. ACobalt hexammine4 perchlorate was precipitated as a pink solid and. tl'ieliquid contained ammonium perchlorate and a small concentration of free ammonia, about .2 normality.

The aqueous suspension was placed in a closed vessel I and carbon dioxide-free air was passed by means of a blower 2 and pipe 3 through it, the nitrogen and undissolved oxygen being allowed to escape through a pipe 4 any ammonia carried away by it, being scrubbed out in the scrubber 25. The contents of the vessel I were stirred by means of a stirrer 5 and prevented from rising in temperature above 30 C. by passing cold water as required from the pipe 6 through the coil 1. The pressure in the apparatus through the whole operation was maintained at substantially atmospheric. After 20.9 parts by weight of air had been passed, the absorption of oxygen became very slow and 2.4 parts by weight of oxygen had been absorbed. i

The liquid was then withdrawn'from the bot; tom of the vessel I through the porous bottom 8 by means of a pump 9, and pumped to the vessel IS. The porous bottom ensured that only liquid was withdrawn, the solid dark brown to black peroxo compound into which the pink cobalt .ingthe oxygen absorption process and from the hexammine perchlorate had changed being re.- 'tained 'in theo-vessel. For the regeneration -of the oxygen from.l the peroxo compound an aqueous solution containing 39 parts by weight'of ammonium` perchlorate, 27.2 parts by weight `of ammonia and 400 .parts by Weight of water was runinto the vessel I from thetank II, through the pipe I6. Ihe resulting suspension of Vthe peroxo compoundfwas then vigorously stirred when oxygen was. evolved and collected via the pipe I4, the valves o-n both pipes f3. and 4 being closed. Any ammonia in the levolved oxygen was scrubbed out by the scrubber l24'. The stirring was kcontinued until 2.4`vvparts Lby weight of oxygen: had been evolvedand Ithe peroxo compound had been completely converted tothe pink cobalt hexammine perchlorate.' 'Dur- 'ing regeneration of oxygen heat` was absorbed and hot'water` was passed through the Acoil 1 by the :pipe 6 to maintain the temperature of vtentsof. the vessel I just below.30. CL..

the con- When regeneration of oxygen was complete the lliquid in the vessel I was withdrawn throughthe liquor to the peroxo compound duringthe'oxygen regeneration, the weak ammonia solutionz'stored .in the tank I0 becamestronger and thestrong .ammonia solution stored in the tank IIibecame `Wea-ker in ammonia. In order to maintain the E0 liquor at about the desired strength, some ofsthe liquor in the tank I0 was pumped. via pipe `I 'Iz and .pump I 8 through a still I9 heated by a steam coil i, 20.1 The resulting ammoniavapour andjsteam was passed `through pipe 22 to a'condenser-23 and l@5 intothe vtank II,.while the stripped liquor from the still Illu-was returned via pipe 2| to the tankl I0. Additional ammonia was added as required,

tolmake up for'zlosse's, through thepipeggnto the tank4 Il; v Y.: Further useful complex cobaltous'salts are co'- balt hexammine chloride, fluoride, bromide, ace-4 tate and nitrate. Instead of the ammonia we could have used other complex-forming radicals such as aniline, toluidne, methylamine, triethanolamine, ethylene diamine, pyridine and ethanolamine, and instead of using water as a liquid medium we could have used methyl alcohol, ethyl alcohol, glycol or formamide.

The process of the present invention is of particular value in recoveringoxygen from air, but it may also be applied -to other oxygen-containing gaseous mixtures provided that gases which are deleterious to the absorbent are either absent or are removed prior to the absorption process.

As many apparently widely diierent embodiments of this invention may made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specic embodiments thereof except as defined in the appended claims.

We claim:

1. A process for the recovery of oxygen from gaseous mixtures containing it, which comprises the employment, Vas absorbent for the oxygen, of

a .complex cobaltous perchlorate capable of exchanging part of its complex-forming radical for oxygen and vice Versa, in association with at least one aqueous medium the absorption and regeneration being effected below the temperature at which the cobaltous perchlorate will be oxidised to a cobaltic perchlorate in any substantial quantity. i

2. A process as claimed in claim 1, in which the absorption is effected by increasing the partial pressure of the oxygen above the aqueous medium 'and regeneration is effected by decreasing the partial pressure of the oxygen above said medium.

. 3. A process as claimed in claim 1, in which the absorption is effected in the presence of at least one aquous medium relatively Weak in free complex-forming radical, and regeneration is effected by increasing the concentration of free complex-forming radical in said aqueous medium.

4. A process as claimed in claim 1 in which the complex cobaltous perchlorate is largely present in the solid phase.

5. A process as claimed in claim 1, in which the complex cobaltous perchlorate is changed to a cobaltous peroxo compound during the absorption step, said peroxo compound being largely present in the solid phase.

6. A process as claimed in claim 1 in which the complex cobaltous perchlorate is sparingly soluble vin 'the aqueous medium.

'7. A process as claimed in claim 1, in which complex cobaltous perchlorate is converted to a cobaltous peroXo compound which is sparingly soluble in the aqueous medium.

8. A process as claimed in claim 1 in which the complex cobaltous perchlorate is cobaltous hexammine perchlorate. v

9. A process as claimed in claim 1 in which the aqueous medium contains a soluble perchlorate.

10. A process as claimed in claim 1 in which the aqueous medium contains a buffer for the cornplex-forming radical.

11. A process as claimed in claim 1 in which the absorption iseiected in the presence of at 'least one aqueous medium relatively Weak in free complex-forming radical, and regeneration is effected by replacing the aqueous medium with one stronger inv free complex-forming radical.

l2. A process as claimed in claim 1, in which the absorption and regeneration is controlled by raising the concentration of free complexforming radical in the aqueous medium afterr the absorption step to elect regeneration, said increase jin concentration being effected by having present a compound capable of giving rise to an increase in the concentration of free complexforming radical With increase in temperature, and increasing said temperature to bring about said effect. Y

l13. A process as claimed in claim 1, in which theabsorption and regeneration is controlled byy raising the concentration of free complexforming radical in the aqueous medium, a hydrolysable saltof the complex-forming radical with an acid which can easily be removed from the aqueous medium and Which salt is capable of giving rise to increases in the concentration of free complex-forming radical with increases in temperature, and raising said temperature to obtain the required concentration of free complex formingradical.

14. A process as claimed in claim 1 in which the complex-forming radical is ammonia and in which the absorption and regeneration is controlled by'varying the concentration of free ammonia in the aqueous medium such variation being eiected by having ammonium borate present in the aqueous medium and varying the temperature.

15. A process as claimed in claim 1 in which the complex-formingradical is ammonia and in which the absorption and regeneration is controlled by varying the concentration of free ammonia in the aqueous medium such variation being eiected by having lithium met-aborate octahydrate present in the solid phase and a soluble ammonium salt present in the aqueous medium and varying the temperature.

16. -A process as claimed in claim 1 in which the complex-forming radical is ammonia and the absorption and regeneration is eiected below 1'7. A process as claimed in claim 1 in which the rate of regeneration is increased by the presence oi a polyhydric alcohol in the aqueous medium.

18. A process as claimed in claim 1 in which theI rate of regeneration is increased by the presence of ethylene glycol in the aqueous medium.

`KENNETH CECIL WARNE. JAMES WILLIAM WOOLCOCK. 

